Put garlic, onion, carrots, oil, and lentils in a big soup pot (I always use a cast iron pot). Add water until its about an inch above the lentils and stuff. Cook on low-medium-low (2 out of 10 on my stove, with 10 being the highest) for a few hours, until everything starts to get mushy. Add spices and cook for another half hour.
I usually wait to add salt to each serving because its easy to add too much.